Cheapest priced copy (Time Spiral): $0.49 at Card Kingdom, near mint, their price list as of .
Opal Guardian 
Enchantment · Rare · Time Spiral · #30
When an opponent casts a creature spell, if this permanent is an enchantment, this enchantment becomes a 3/4 Gargoyle creature with flying and protection from red.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Time Spiral | 30 | Rare | $0.49 | $0.79 |
4 rulings from Wizards of the Coast
- 25 September 2006After the triggered ability resolves, Opal Guardian is no longer an enchantment.
- 25 September 2006Guardian's effect doesn't wear off.
- 25 September 2006A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).
- 1 August 2008A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.